The present invention relates to a system and method for extracting heat from hot unrefined water for the purpose of utilizing this heat to do useful work, and more especially, to a system and method for extracting heat from water from a geothermal source wherein the disadvantageous effects upon the heat extraction system as a result of scaling and other solid build-up of impurities contained in the unrefined hot water are substantially avoided.
In the course of the presently ensuing search for additional and improved sources of energy to meet rapidly growing demands, investigators are studying the feasibility of utilizing naturally available energy sources such as, for example, naturally heated water from geothermal sources. This source of energy provides the additional advantage that it is nearly pollution free, since after absorption of its heat, the water can be returned to the ground leaving no polluting by-products.
One of the most significant problems, however, associated with extraction of useful heat from geothermically heated water resides in the fact that the water often contains large amounts of impurities, both in solution and in suspension. This water is referred to herein as unrefined water and is often saturated or supersaturated with impurities such as silica, calcium sulfate, silicates and other compounds. Other impurities such as silica, silicates and iron may be present in the form of colloidal dispersions in the water. These impurities give rise to severe problems of scaling, corrosion, etc., on the surfaces of the apparatus utilized to transport the water from the underground source and, even more particularly, the heat exchange apparatus utilized for extracting heat from the water. Purification of the unrefined water prior to extracting its heat is unreasonably expensive and decreases the efficiency of geothermal power sources.
In co-pending U.S. patent application Ser. No. 424,470, of which this application is a continuation-in-part, there is described a novel system and method for extracting heat from hot unrefined water with the avoidance of detrimental effects caused by impurities contained in the water. According to the disclosure of that application, the hot unrefined water is prevented from coming into direct contact with the surfaces of the heat exchanger used to boil a working fluid. Filters and like equipment are not needed. A heat transfer medium in the form of a housing containing porous material such as a bed of gravel or other granular material is used to transfer heat from the unrefined water to clean water which is then passed through the heat exchanger. Such a heat transfer medium will be referred to herein as an accumulator-type heat interchanger. The porous material is inexpensive and expendable and can even be easily cleaned and reused if desired.
In the system a volume of the hot unrefined water is passed through a housing containing porous material which picks up the heat of the water. A volume of clean water is then passed through the housing to pick up the heat from the porous material. The now heated clean water can then be passed through a heat exchanger without significant danger to the surfaces of the exchanger. The clean water can be recycled through the system many times, each time passing through the housing immediately after a volume of the unrefined water.
In another important aspect of the system disclosed in the co-pending application, the source of the clean water may be the unrefined water which has been passed through the porous material. After being removed from the housing, the cooled unrefined water is delivered to a detention receptacle. Here it attains stabilization as many of the impurities settle to the bottom of the receptacle. The liquid which is left on the top of the receptacle is substantially free of impurities to the extent that what impurities are left in the liquid are not sufficient to unduly damage the surfaces of the heat exchanger. It is this substantially impurity-free liquid which is used as the clean water, yet no filtering, etc., is necessary.
A preferred embodiment of the system disclosed in the co-pending application provides for continuous operation of the system by the use of two housings containing porous material. The source of hot unrefined water is connected to the entrance end of one of the housing and hot unrefined water is passed through this housing until the leading edge of this volume of water is at the exit end of the housing. At this time, the source of hot unrefined water is disconnected from the first housing and connected to the entrance end of the second housing; simultaneously the source of cool, clean water is connected to the entrance end of the first housing (having been previously connected to the second). At the same time that the source of clean water is connected to the first housing, i.e. when the leading edge of the preceding volume of unrefined water has reached the exit end of the first housing, the exit end of the first housing is connected to the detention receptacle so that the unrefined water may be deposited therein. Meanwhile, the leading edge of a volume of clean water, which water has been heated, has reached the exit end of the second housing which is then connected to the heat exchanger. Thus, the entrance ends of the housings are alternately connected to the sources of unrefined and clean water and each time the connections at the entrance ends are switched, the connections at the exit ends are also switched to alternately direct unrefined and clean water from the housings to the detention receptacle and the heat exchanger respectively. To allow for proper timing in this simultaneous switching of the connections of the entrance and exit ends of the beds, a temperature front, on one side of which the porous material and water are at their highest temperature and on the other side of which the porous material and water are at their lowest temperature, must move along the porous material with half the velocity of the water volume such that it is at the center of the housing when the leading edge of a volume of water has reached the exit end. One way of achieving this is to choose the porous material such that its heat capacity per unit volume when dry is substantially the same as that of the unrefined water or clean water contained in the voids of a unit volume of the gravel or other porous material.
Further details of the system and method disclosed in the co-pending application will be understood from the disclosure of that application which is hereby specifically incorporated by reference and is relied upon.
It was discovered in accordance with the invention disclosed in the co-pending application that the small concentration of particles remaining in suspension in the substantially impurity-free liquid withdrawn from the detention receptacle as a source of clean water for the heat exchanger gives rise to a beneficial effect, namely, their presence inhibits the formation of scale upon the surfaces of the heat exchange apparatus. This phenomenon is believed to be the result of several interacting factors. The presence of a large number of small suspended or dispersed particles provides a large surface area closely associated with impurities still dissolved in the water, so that upon cooling the water in the heat exchange apparatus impurities tending to precipitate will preferentially precipitate upon the surface area provided by the suspended or dispersed particles as opposed to the surfaces of the heat exchange apparatus. This is especially true where the particles are of the same chemical constituency as the dissolved impurities, due to a seeding effect. Furthermore, where the dispersed particles are of the same constituency as the dissolved impurities tending to precipitate, the particles will also be of the same nature and charge as any material which has deposited in the heat exchange apparatus. Thus, the dispersed particles will show a reduced tendency to associate with deposits in the apparatus.
While the method and apparatus described in the copending application provide a means for extracting heat from hot unrefined water which avoids the significant problem of scaling in the heat exchange apparatus, it is still desirable to improve the efficiency of this method and system, e.g. to decrease the amount of deposits upon the porous material used in this system. Furthermore, it is desirable to seek means for preventing undesirable deposits in the well casing and piping employed for bringing water from an underground source to the heat exchange apparatus. Similarly, it would be desirable to limit the danger of precipitation of impurities from hot unrefined water to the extent that the unrefined water could be passed directly into conventional heat exchange apparatus, without the necessity of subjecting the water to conventional purification techniques which are expensive and inefficient.